Transgenic plants have been an integral component of advances made in agricultural biotechnology. They are necessary tools for the production of plants exhibiting desirable traits (e.g. herbicide and insect resistance, drought and cold tolerance), or producing products of nutritional or pharmaceutical importance. As the applications of transgenic plants become ever more sophisticated, it is becoming increasingly necessary to develop strategies to fine-tune the expression of introduced genes. The ability to tightly regulate the expression of transgenes is important to address many safety, regulatory and practical issues. To this end, it is necessary to develop tools and strategies to regulate the expression of transgenes in a predictable manner.
Several strategies have so far been employed to control plant gene/transgene expression. These include the use of regulated promoters, such as inducible or developmental promoters, whereby the expression of genes of interest is driven by promoters responsive to various regulatory factors (Gatz, 1997). Other strategies involve co-suppression (Eisner et al., 1998) or anti-sense technology (Kohno-Murase et al., 1994), whereby plants are transformed with genes, or fragments thereof, that are homologous to genes either in the sense or antisense orientations. Chimeric RNA-DNA oligonucleotides have also been used to block the expression of target genes in plants (Beetham et al., 1999; Zhu et al., 1999).
Posttranslational modifications of histones in chromatin are important mechanisms in the regulation of gene expression. Protein-protein interactions between histones H3, H4, H2A and H2B form an octomeric core which is wrapped with DNA. N-terminal tails of histones protrude from the octamer and are subject to posttranslational modification involving acetylation and deacetylation of conserved lysine residues. A nucleosome comprises 26 lysine residues that may be subject to acetylation. Acetylation of core histones, including H4 and H3 via histone acetyltransferase (HAT), is correlated with transcriptionally active chromatin of eukaryotic cells. Acetylation is thought to weaken the interactions of histones with DNA and induce alterations in nucleosome structure. These alterations enhance the accessibility of promoters to components of the transcription machinery, and increase transcription. HATs have been identified in yeast, insects, plants and mammals (e.g. Kolle et al. 1998), and are typically components of multiprotein complexes including components of RNA polymerase II complex, TFIID, TFIIC and recruitment factors (e.g. see Lusser et al. 2001 for review).
Histone deacetylation, via histone deacetylase (HD, HDA, HDAC), is thought to lead to a less accessible chromatin conformation, resulting in the repression of transcription (e.g. Pazin and Kadonaga, 1997; Struhl, 1998; Lusser et al., 2001). The role of the yeast histone deacetylase, RPD3, in transcriptional repression was first discovered through a genetic screen for transcriptional repressors in S. cerevisiae (Vidal and Gaber, 1991). Since then, a number of yeast and mammalian HDAC genes have been cloned (Rundlett et al., 1996; Emiliani et al., 1998; Hassig et al., 1998; Verdel and Khochbin, 1999). Most eukaryotic histone deacetylases show some sequence homology to yeast RPD3, suggesting that these proteins are all members derived from a single gene family (Khochbin and Wolffe, 1997; Verdel and Khochbin, 1999). In yeast and mammalian cells, the RPD3/HDACs mediate transcriptional repression by interacting with specific DNA-binding proteins or associated corepressors and by recruitment to target promoters (Kadosh and Struhl, 1997; Hassig et al., 1997; Nagy et al., 1997; Gelmetti et al., 1998). Recently, a second family of histone deacetylases, HDA19 and related proteins, were identified in yeast and mammalian cells (Rundlett et al., 1996; Fischle et al., 1999; Verdel and Khochbin, 1999). The deacetylase domain of HDA19-related proteins is homologous to but significantly different from that of RPD3 (Fischle et al., 1999; Verdel and Khochbin, 1999). These proteins also appear to be functionally different from RPD-like proteins in yeast cells (Rundlett et al., 1996). WO 97/35990 discloses mammalian-derived histone deacetylase (HDx) gene sequences, gene products, and uses for these sequences and products. The down regulation of gene expression in plants using histone deacetylase, fused to a DNA binding domain that targeted the fusion protein to a specific gene, has been demonstrated (Wu et al., 2000a; Wu et al., 2000b).
The present invention embraces the use of fusion proteins comprising a DNA binding domain fused to a recruitment factor, that is capable of recruiting chromatin remodelling proteins such as HDAC and HAT, to specific DNA sites to regulate expression of a gene of interest. Also disclosed is the use of fusion proteins comprising a DNA binding portion fused to histone acetyltransferase (HAT) to regulate transcription of a gene of interest.